Fig. 1. Typical PMT configuration^[30]. (a) Dynode PMT; (b) microchannel PMT

Fig. 2. Structure of typical Si-based SPAD^[32]

Fig. 3. Structure and working mechanism of typical SNSPD^[35]. (a) Typical structure of SNSPD; (b) hotspot model

Fig. 4. Lidar configuration based on frequency upconversion SPD^[42]

Fig. 5. Configuration of SPD. (a) Block diagram of the nuclear data acquisition and control system^[47]; (b) block diagram of SPD^[48]

Fig. 6. Components' design and inversion profiles. (a) Experimental setup of CO₂ DIAL^[49]; (b) vertical CO₂ number density profile and height of retrieved boundary layer^[49]; (c) schematic of frequency up-conversion single photon detection atmospheric communication^[50]

Fig. 7. Schematic diagram of lidar and design diagram of superconducting nanowires based on SNSPD^[55]

Fig. 8. Configuration of T2 lidar and measured signal^[56]. (a) Photo of exterior and interior of T2 lidar, and flow chart of radar components; (b) raw signal and range-corrected photons received by T2 lidar operated in continuous mode

Fig. 9. Observation of cloud vertical structure^[57]. (a) Distribution of observed photons; (b) fraction F(i) of observed photons within each sublayer i observed for four selected time-gated windows in (a); (c) profile of observed photon rate within first 1.5 m of each time-gated window

Fig. 10. The system and obtained curves of lidar^[3]. (a) System layout of lidar; (b) picture of up-conversion single-photon detector;(c) backscattering signal of 24 h continuous horizontal detection starting at 13∶00 on Oct. 26, 2014 and an example of smoke detected over 1 h; (d) measured results, from top to bottom, are extinction coefficient, visibility, humidity, and atmospheric temperature near ground

Fig. 11. Forty-eight-hour observation of atmospheric wind and visibility. (a) Wind speed^[15]; (b) wind direction^[15]; (c) visibility, temperature, and humidity^[15]; (d) contrast visibility^[59]; (e) measured wind speed and direction^[59]

Fig. 12. Schematics and inversion spectra of lidar. (a) Schematic of dual-frequency Doppler lidar for wind detection^[60]; (b) raw lidar signals, zonal wind, meridional wind, horizontal wind, and wind direction at 17∶45, on March 15, 2017^[60]; (c) optical layout of polarization lidar^[61]; (d) raw signal of polarization lidar over 48 h and 48 h continuous measurement results of linear depolarization ratio^[61]

Fig. 13. Design and inversion data of lidar^[63]. (a) Schematic diagram of lidar system; (b) time averaged altitude profiles of backscattered signals from 0:00 to 0∶15, on Jan. 2 in 2022; (c) comparison of backscatter ratio retrieved from Klett-Fernald method (red straight line: using 1064 nm lidar data; black dotted line: using 532 nm lidar data) and dual-wavelength method (blue dashed line), the profiles are depicted from the experiment conducted from Jan.1 to Jan. 2 in 2022, and each profile is accumulated in 15 min; (d) backscatter signal of 532 nm and 1064 nm lidar, aerosol backscatter ratio retrieved using 1064 nm lidar data and an enlarged illustration of the aerosol layer around 25 km, the time resolution is 3 min and the altitude resolution is 90 m

Fig. 14. Ranging principle and acquired map^[69]. (a) Schematic diagram of principle of satellite single-photon laser ranging; (b) map of average annual rate of change in glacier elevation

Fig. 15. Distribution of the first particle optical quantum radar network sites^[70]

Fig. 16. Photos and inversion curves of aerosol lidar^[71]. (a) Schematic diagram and physical maps of up-conversion atmospheric aerosol lidar; (b) time-height plot of atmospheric boundary layer and cirrus clouds

Fig. 17. Detecting mechanism and installation diagram^[74]. (a) Schematic diagram of compact space-borne lidar detection principle; (b) installation diagram of compact space-borne lidar

Fig. 18. Echo signals obtained from 8 h of the compact space-borne lidar detection on Dec. 19 in 2022^[74]. (a) 532 nm P channel; (b) 532 nm S channel; (c) 1064 nm channel